This invention relates generally to digital transmission systems and particularly to a digital data transmission system in which data may be transmitted and received using any one of a plurality of different data constellations. The invention more specifically concerns a system for communicating information to a receiver regarding the particular data constellation being used at any given time in a respective channel.
U.S. Pat. No. 5,087,975 discloses a vestigial sideband (VSB) system for transmitting a television signal (or other data signal) in the form of successive M-level symbols over a standard 6 MHz television channel. The television signal may, for example, comprise one or two compressed wideband HDTV signals or a number of compressed NTSC signals. While the number of levels M characterizing the symbols may vary depending on circumstances, the symbol rate is preferably fixed, such as at a rate of 684 H (about 10.76 Megasymbols/sec), where H is the NTSC horizontal scanning frequency. The number of symbol levels used in any particular situation (hereinafter referred to as the VSB mode) is largely a function of the S/N ratio characterizing the transmission medium, a smaller number of symbol levels, i.e. a lower VSB mode, being used in situations where the S/N ratio is low. It is believed that the ability to accommodate symbol levels of 24, 16, 8, 4 and 2 provides adequate flexibility to satisfy conditions in most systems. It will be appreciated that lower values of M can provide improved S/N ratio performance at the expense of reduced transmission bit rate. For example, assuming a rate of 10.76 M symbol/sec, a 2-level VSB signal (1 bit per symbol) provides a transmission bit rate of 10.76 Megabits/sec, a 4-level VSB signal (2 bits per symbol) provides a transmission bit rate of 21.52 Megabits/sec and so on up to a 24-level VSB signal which provides a transmission bit rate of about 48.43 Megabits/sec.
In a system of the foregoing type, each receiver must be informed of the VSB mode in use at any given point in time on the channel to which it is tuned. This is preferably accomplished by multiplexing a VSB mode control signal with the transmitted data which can be extracted by each receiver tuned to the respective channel, the VSB mode control signal identifying the number of levels M characterizing the symbols in use on the channel. In order to provide appropriate operating characteristics, it is desired that the receivers be capable of quickly acquiring the VSB mode control signal after a channel change and that the probability of the so-acquired signal being correct is very high. Also, the receivers must be capable of acquiring, although not necessarily as quickly, a new VSB mode control signal without an intervening channel change. At the same time, of course, a capability must be provided to replace an acquired incorrect mode control signal. Finally, once acquired, a correct mode control signal should be virtually immune to noise bursts and the like.